# Junior Cert Science: Unusual demonstration of magnetic fields

This week we looked at a concept called ferromagnetism which allows us to demonstrate the 3-dimensional nature of magnetic fields.

Thanks to my first years for being so co-operative 🙂

Here’s another way of illustrating the idea (which we didn’t do)

# Leaving Cert Physics: Unusual resonance demonstrations

This week we demonstrated resonance.

Apart from the standard demonstrations using tuning forks and wine glasses we also tried out variations

1. The Chinese Bowl

2. The aluminium rod

# How to get 100% in your Leaving Cert Physics exam. Part 2: Answering Graph Questions

Drawing the graph

• You must use graph paper and fill at least THREE QUARTERS OF THE PAGE.
• Use a scale which is easy to work with i.e. the major grid lines should correspond to natural divisions of the overall range.
• LABEL THE AXES with the quantity being plotted, including their units.
• Use a sharp pencil and mark each point with a dot, surrounded by a small circle (to indicate that the point is a data point as opposed to a smudge on the page.
• Generally all the points will not be in perfect line – this is okay and does not mean that you should cheat by putting them all on the line. Examiners will be looking to see if you can draw a best-fit line – you can usually make life easier for yourself by putting one end at the origin. The idea of the best-fit line is to imagine that there is a perfect relationship between the variables which should theoretically give a perfect straight line. Your job is to guess where this line would be based on the available points you have plotted.
• Buy a TRANSPARENT RULER to enable you to see the points underneath the ruler when drawing the best-fit line.
• DO NOT JOIN THE DOTS if a straight line graph is what is expected. Make sure that you know in advance which graphs will be curves.
• BE VERY CAREFUL drawing a line if your ruler is too short to allow it all to be drawn at once. Nothing shouts INCOMPETENCE more than two lines which don’t quite match.
• Note that examiners are obliged to check that each pint is correctly plotted, and you will lose marks if more than or two points are even slightly off.
• When calculating the slope choose two points that are far apart; usually the origin is a handy point to pick (but only if the line goes through it).
• When calculating the slope DO NOT TAKE DATA POINTS FROM THE TABLE of data supplied (no matter how tempting!) UNLESS the point also happens to be on the line. If you do this you will lose beaucoup de marks and can kiss goodbye any chance of an A grade.

What goes on what axis?

Option one

To show one variable is proportional to another, the convention is to put the independent variable on the x–axis, and the dependant variable on the y-axis, (from y = fn (x), meaning y is a function of x). The independent variable is the one which you control.

Option two

If the slope of the graph needs to be calculated then we use a difference approach, one which often contradicts option one, but which nevertheless must take precedence. In this case we compare a formula (the one which connects the two variables in question) to the basic equation for a line: y = mx.

See if you can work out what goes on what axis for each of the following examples (they get progressively trickier):

1. To Show Force is proportional to Acceleration
2. Ohm’s Law
3. Snell’s Law
4. Acceleration due to gravity by the method of free-fall
5. Acceleration due to gravity using a Pendulum

There is usually a follow-up question like the following;

“Draw a suitable graph on graph paper and explain how this verifies Snell’s Law”.

There is a standard response to this;

“The graph of Sin i against Sin r resulted in a straight line through the origin (allowing for experimental error), showing Sin i is directly proportional to Sin r, and therefore verifying Snell’s Law”.

If you are asked any questions to do with the information in the table, you are probably being asked to first find the slope of the graph, and use this to find the relevant information.

# How to get 100% in your Leaving Cert Physics exam. Part 1: Section A

Section A counts for 30% of your overall mark and is the easiest section to pick up full marks. There are about 24 experiments but many of them are minor variations on each other. Stop wasting time trying to predict which ones will come up and just learn them all. Take one or two per night and make sure you can answer every question on each experiment from past papers. In particular you need to use the following as a checklist for each experiment.

(i)     Draw a fully labelled diagram which includes all essential apparatus (have you included the apparatus necessary to obtain values for both variables?).

(ii)   Be able to state how the two sets of values were obtained (this is a very common question).

(iii) Describe what needs to be adjusted to give a new set of data

(iv) Write down the relevant equation if there is one associated with the experiment.

(v)   Be able to state how the data in the table will need to be adjusted.

(vi) Know what goes on each axis.

(vii)           Know how to use the slope of the graph to obtain the desired answer.

(viii)         Be able to list three sources of error/precautions.

Misc Points

• The graph question is usually well worth doing.
• Learn the following line off by heart as the most common source of error: “parallax error associated with using a metre stick to measure length / using a voltmeter to measure volts etc”.
• Make sure you understand the concept of percentage error; it’s the reason we try to ensure that what we’re measuring is as large as possible.
• There is a subtle difference between a precaution and a source of error – know the distinction.
• When asked for a precaution do not suggest something which would result in giving no result, e.g. “Make sure the power-supply is turned on” (a precaution is something which could throw out the results rather than something which negates the whole experiment).
• To verify Joule’s Law does not involve a Joulemeter
• To verify the Conservation of Momentum – the second trolley must be at rest.
• To verify the laws of equilibrium – the phrase ‘spring balance’ is not acceptable for ‘newton-metre’.
• To measure the Focal length of a Concave Mirror or a Convex Lens.
Note that when given the data for various values of u and v, you must calculate a value for f in each case, and only then find an average. (As opposed to averaging the u’s and the v’s and then just using the formula once to calculate f). Apparently the relevant phrase is “an average of an average is not an average”.

I have a document here which containts exam questions on every experiment which has ever appeared on a past paper from 2002 to 2010 (Higher Level and Ordinary Level) – this should be your bible for Section A over the coming weeks. Solutions are also included.

Now get back to work.

More to come.

# Imagine if the key-word in the Leaving Cert Physics syllabus was ‘wonder’

This is an image, courtesy of Wordle.net, of the current Leaving Certificate Physics syllabus. Wordle is a program that gives the most common words the largest font size:

This is a similar image of the proposed new syllabus.

Notice the new focus on the words ‘learners’ and ‘learning’.

Imagine if a syllabus had as its most common words the following:

Engage

Passion

Awe

Curiousity

Inspire

Un-nerve

Emotion

Story

Creativity

Wonder

If any of this was a priority then chances are that Particle Physics wouldn’t have been removed (and with it Pair Annihilation, Anti-matter, Neutrinos, Fundamental Forces, etc.).

Chances are that Cosmology would also feature strongly in the new syllabus (Black Holes, Quasars, Pulsars, Big Bang, Neutrinos (again), Dark Matter, Alien Life, etc. etc.). It doesn’t.

Maybe it’s just me.

# A short history of the term ‘Voltage’

Image created with wordle.net

Potential difference (commonly known as ‘voltage’) is probably one of the least well understood concepts in Leaving Cert Physics.

Not only is it difficult to understand such an abstract concept (it’s not like you can hold a bunch of voltage in your hand), but the meaning of the term ‘voltage’ has itself changed over the years, and you’re left to make sense of the remaining muddle.

If you think I’m just saying this to make you feel better, read on.

The following extract has been taken from the minutes of a History of Science meeting in 2002.

John Roche, of Linacre College, Oxford, opened the session after tea, speaking on the concept of voltage. He began by claiming that almost every concept in electricity and electromagnetism is ambiguous, and the concept of voltage is one of the most incoherent. Its evolution in difficult to follow.

Abbé Nollet, in the 18th century, distinguished quantity and degree of electrification. Others made similar distinctions between quantity and intensity or tension or pressure – what we would call voltage.

Roche showed how the term “voltage” had come to be used nowadays in three different ways; for electromotive force, potential difference and (absolute) potential.

Volta defined electrical tension as the endeavour of the electrical fluid to escape from a body. Volta’s tension was more akin to a force, unlike the modern definition of electromotive force, which is a misnomer, being defined in terms of energy.

Ohm carried Volta’s concept to closed circuits with the idea that voltage was proportional to the difference in tension between the ends of a conductor. For Ohm, it was the gradient of electrical tension that drove the current.

Poisson introduced an entirely different concept, of charge divided by distance to a point, which Green called the potential. This was an analytical device only, arising from an analogy with Laplace’s gravitational potential function.

Kirchhoff reconciled Volta’s tension with Poisson’s potential function through the concept of energy or vis viva introduced by Helmholtz. From Kirchhoff, current is driven by the electric field in a conductor and voltage is related to the energy supplied, but physicists and electrical engineers do not usually think of them in this way.

All the earlier interpretations remain current, but with different weights, and most of the time voltage is seen as a driving energy.

IOP History of Physics Group Newsletter, Spring 2000, page 65

With thanks to @draziraphale for prompting this post

# Physics subject plan online

We had a Physics inspection last year which involved putting together a Subject plan (not that it didn’t exist previously of course, but you know what I mean). It did take quite a bit of time and it occurred to me that, given that most schools in the country offer Physics as a subject, it would make sense to those who have yet to create their own to see one which is already in existence.
It is of course not the definitive article but nevertheless should prove particulary useful to both new teachers and those who have been ‘drafted in’ from other subjects to teach Physics.

Feel free to incorporate as much or as little of it as you like; if I make any major changes I will let you know via the blog or twitter.

The plan itself can be accessed directly from here or from the homepage of thephysicsteacher.ie

Feel free to offer constructive critisism 🙂

I hope to get around to producing something similar for Applied Maths at a later stage.

Wouldn’t it be nice if every department in every school had to put their subject plans online – I’m sure it would be appreciated by parents who are looking to decide what school to send their children to.

# Quantum Theory – why do we ignore the mystery?

Isn’t it crazy that one of the most wonderful concepts in Physics – the dual nature of light – doesn’t get a better deal from the leaving cert physics syllabus?
Students are expected to know how to demonstrate that light is a wave, and also to be able to recall Einstein’s interpretation of the Photoelectric effect (which proved that light is a particle) but then there is nothing else about what is one of the greatest mysteries in Physics – how can light be both particle and wave?
Quantum Theory is one of the most popular concepts in popular science books, yet we leave it out altogether.
Isn’t there a responsibility on us as teachers to make our voices heard? Or is it the case that we don’t really care?
The following is a video taken during the Solvay Institute of 1927 – it helps to give some feel for the characters involved (see the Quantum Physics page of thephysicsteacher.ie for a link to this and other related videos).

This is one of my favourite videos on quantum theory – it emphasises the wonder, and that’s always a cool trick when introducing any new physics concept to students.

# Nuclear Physics Resouces

For anyone out there interested in educating themselves on all things nuclear. I teach this as two seperate chapters for Leaving Cert Physics. I would like to think that by studying the notes on the two chapters plus watching the associated links you would actually be in a position to answer any leaving cert questions on the topic.

You can download the word documents for the two chapters here (they’re chapters 30 and 31) and the links for the associated videos are beside them but I’ll put them here for convenience:
Fission,Fusion and NuclearEnergy

The nice thing (I think) is that the word documents contain every exam question that has ever appeared on that topic (broken into individual short questions) plus the solution to each question.
At least I think that’s cool.

It means not only can you put yourself in a postion to understand what’s happening in Japan at the moment but it should feel nice to know that you can do leaving cert Physics!

After all, it’s not rocket science (is it?)

Apart from that, the following video serves as a reasonably good source of information although it is by no means comprehensive and doesn’t list the potential dangers, which is what most people want to know.

Finally, a useful timeline of events is available from Mr Reid’s blog here

Normally we cover this topic towards the end of sixth year but this is the perfect time to introduce the concepts to fifth years; I wonder how many teachers chose not to alter their lesson plan because that’s just not the order in which it’s meant to be taught?

# The origin of the decibel scale

A standard leaving cert physics exam question is “why do we have the decibel scale”?
The standard answer is that the range of sound intensities is so large that a second, much more compact scale is required to make the numbers more manageable, and for sound this scale is based on multiples of ten and is called the decibel scale (and what it measures is called sound intensity levels).

The old syllabus included a detailed analysis of this scale so that the numbers actually meant something. For the new syllabus (2002 onwards) it must have been decided that the maths was too difficult so this part was scrapped, except for one very odd ‘fact’; the student must know that a doubling of the sound intensity results in an increase of sound intensity level of  3 dB. Now needing to know that piece of useless trivia is ridiculous and is probably only there as a sop to some university professor who was horrified that the detailed analysis was dropped:

At least that’s my best guess, which doesn’t seem too dissimilar to what the author of a recent book on Physics and Music  entitled How Music Works thinks about the decibel in general.

I think the decibel was invented in a bar, late one night, by a committee of drunken electrical engineers who wanted to take revenge on the world for their total lack of dancing partners.

Ouch!

Now what’s the betting that students will remember this explanation and forget all about the technical one?